The primary assimilation of carbon dioxide into organic matter is catalyzed by the most abundant protein found on earth, D-ribulose 1,5-disphosphate (RuBP) carboxylase. Under suitable conditions, the enzyme may also behave as an internal monooxygenase and catalyze an oxygen fixation reaction. The same polypeptide chain catalyzes both carboxylase and oxygenase activities, making this enzyme one of the few proteins known to be capable of catalyzing two distinct reactions. Moreover, the separate activities represent the first reactions of two competing pathways in cellular metabolism. This study thus represents an unusual opportunity to relate control at the enzymic and molecular level to significant metabolic events. Our recent work has shown that it may be possible to differentially regulate each activity. Thus, part of the proposed work will seek to extend these studies, primarily with a structurally simple bacterial enzyme, with the goal of selectively altering or modifying one activity to the exclusion of the other. Toward this end, we have made the significant observation hat a yellow, fluorescent "cofactor" may participate in only the oxygenase reaction. We will isolate, purify and characterize this material and detemine its role in the oxygenase reaction. Moreover, using techniques of protein chemistry, affinity labeling, and enzyme kinetics, we will continue to investigate the nature of the active site and the regulator sites of this bifunctional enzyme. Lastly, we will investigate the synthesis and assembly of RuBP carboxylase/oxygenase, since this enzyme may comprise up to 50 percent of the soluble protein of relevant bacteria and eucaryotes, where it is localized in discrete organelles.